1. Field of the Invention
The present invention relates to an image forming method for developing a latent electrostatic image and a toner for developing a latent electrostatic image with electrophotography. More specifically, the present invention relates to an image forming method and a toner for developing a latent electrostatic image which enables high-speed two-component development.
2. Description of the Related Art
Conventionally, in image forming apparatus using electrophotography, such as printers or copying machines, from the viewpoint of image quality, durability and high-speed responsiveness, it is desirable to use a two-component developer containing a toner and a carrier. In a two-component development system of this kind, in order to ensure sufficient image density and to improve the reproducibility of fine lines, a developing method is used in which a magnetic brush of developer is placed in contact with a photoconductor, and the circumferential speed of the development sleeve is made faster than the circumferential speed of the photoconductor.
On the other hand, advances have recently been made in respect of achieving full color images, systematization and digitalization in the field of electrophotography, and there have also been demands for higher quality in the output image, faster output speed, and greater stability. The expansion of copying machines and various types of printers into the on-demand printing market is expected. In order to break into the printing market using the electrophotographic method generally employed in copying machines and printers, it is necessary to achieve high quality and high stability, even in the case of long-term, high-speed processing output. An image forming apparatus of this kind, in which a photoconductor and a development sleeve are rotated at high speed and the output of images suitable for the printing market is continued over a long period of time, differs greatly from an image forming apparatus having a medium speed of rotation, which has a normal stirring history and stirring frequency of the developer agent inside the developing unit, and more specifically, a normal continuous contact frequency between the toner and the carrier. Furthermore, these differences become even more pronounced as the conveyance path of the developer becomes longer, due to increase in the size of the development apparatus as a result of increase in the printing speed.
In an ultra-high-speed image forming apparatus which has extremely high continuous contact frequency between the toner and the carrier in this way, charging up of the developer occurs due to the high continuous contact frequency, leading to problems of decline in the image density, and therefore the object of achieving good image stability cannot be attained.
In order to suppress this charging up phenomenon, Japanese Patent Application Laid-Open (JP-A) Nos. 11-231567 and 2001-209209 use a toner formed by adding two types of silica having different degrees of hydrophobization, externally, to the surface of toner. However, in the toners proposed in these references, the difference in the degree of hydrophobization between the two types of silica which are added externally to the toner particles is small and therefore it is not possible to suppress charging up in a high-speed image forming apparatus. Furthermore, in JP-A No. 2006-072093, two different types of silica which are balanced in terms of the difference in the degree of hydrophobization and the difference in particle size are added externally to the surface of toner, but this is not sufficient to suppress the charging up which occurs in an ultra-high-speed image forming apparatus.
On the other hand, with regard to the binder resin used in the toner, in order to achieve high image luster in a full color image, as well as good color reproduction, and a broad fixing temperature range, JP-A No. 61-007844 uses a polyester resin as the binder resin, and JP-A No. 2003-173045 uses a polyol resin. However, if the prior option, the polyester resin, is used, then in an ultra-high-speed image forming apparatus, aggregated material is liable to occur inside the toner bottle or the developing unit, in particular, and a phenomenon of image loss (blank areas) occurs in the portions where the aggregated material is present. Furthermore, if the acid number of the polyester resin is high, then since ambient changes are liable to occur in a low-temperature and low-humidity environment, the charging up phenomenon is encouraged and image density declines. If the latter option, namely, the polyol resin, is used, then although the generation of aggregated material is suppressed, there is some moisture absorption in high-temperature and high-humidity environments, due to the effects of the OH groups in the polymer chain, and consequently a charging down phenomenon occurs and problems of toner scattering and background smear, and the like, arise. Therefore, it is not possible to achieve the object of improving image quality.